Recently, a novel cell type has been described as an endogenous modulator of immune responses in the context of cancer immunity, infectious diseases and allogeneic HSCT(4). The so called ‘myeloid-derived suppressor cells’ (MDSC) have been shown to be able to mediate immune suppression. They are derived from the myeloid lineage and represent a heterogeneous population of myelomonocytic and granulocytic origin. So far these cells have been studied mainly in murine models and their functional role in the human setting of allogeneic HSCT has not been elucidated. The major focus of the current rotation project and the task of the applicant is to measure the frequency of the recently described MDSC subset of granulocytic origin in various graft sources (mobilized blood, bone marrow and cord blood) and in the recipient at defined time-points after allogeneic HSCT. Therefore a multiparameter FACS protocol will be developed to quantify CD11c++/CD62Ldim/CD11b++/CD16++ neutrophil subsets before and after allogeneic HSCT. Their frequency in longitudinal analyses will be correlated with i) the frequency of immune effector cells (CD4+/CD8+ T cells, NK cells, B cells and Tregs) ii) the graft source and iii) clinical parameters after allogeneic HSCT including the occurrence of organ toxicity, infection and GvHD. Finally, neutrophilic MDSC will be isolated by FACS and their DNA will be isolated in order to quantify the percentage of donor cells at given time-points after allogeneic HSCT. The sorted neutrophilic MDSC subset will tested in-vitro for its suppressive capacity against activated CD4+ and CD8+ T cells. This will include the intracellular staining for Interferon- and IL-4 in CD4+ cells after stimulation. Moreover, we will attempt to identify functional indicators of MDSC activity. In particular, the expression levels and more importantly the activation status of adhesion molecules, such as LFA-1 (CD11a/CD18), Mac-1 (CD11b/CD18) and VLA-4/-5 on this cellular subset will be assessed. This will further include the analysis of the physical interaction of MDSC with T cells. Another specific aim is to analyze whether this subset of MDSC is mobilized by G-CSF in healthy donors and with CXCR-4 antagonists in cancer patients and/or donors.